Value documents will be understood here to mean sheet-shaped objects that represent for example a monetary value or an authorization and, hence, should not be producible arbitrarily by unauthorized persons. Hence, they have features that are not easily produced, in particular copied, whose presence is an indication of authenticity, i.e. production by an authorized body. Important examples of such value documents are coupons, vouchers, checks and in particular bank notes.
On account of the great number of value documents in circulation and the need to check them, there exist value document processing apparatuses that serve to check the value documents by machine, for example for their authenticity, fitness for circulation or denomination. For this purpose, value document processing apparatuses frequently have checking devices for checking value documents according to given criteria, which can in particular possess optical sensors. The checking devices and in particular optical sensors of such checking devices are in general sensitive to soiling, in particular dust which can settle on parts of the checking device, for example a window of an optical sensor through which optical radiation passes to or from the value document. As of a certain degree of soiling, the checking device must be cleaned, which requires effort and limits the possible operating time of the value document processing apparatus.